Many of today's highly sensitive electronic components, such as computer and computer-related equipment, that are used in commercial and residential applications contain transient voltage surge suppression (TVSS) devices. These devices protect sensitive and/or expensive electronic circuits and components from damage from over-voltage fault conditions. Such transient voltage surge suppression systems are typically designed for moderate fault conditions expected in normal use. In this respect, such systems are designed to suppress relatively minor fault conditions, but are not designed to protect against major over-voltage conditions. Examples of major over-voltage conditions include those that may occur from losing the system neutral or ground termination, or from repetitive current pulses as from lightning strikes. Such major over-voltage conditions can have catastrophic effects on sensitive electronic circuits and components. To prevent such fault conditions from reaching and damaging electronic circuits, components and equipment, it has been known to utilize larger voltage surge suppression devices. These devices are typically deployed at a building's incoming electrical service power lines, or within a building's power distribution grid to control power surges in the electrical lines to the building, or in the electrical lines to specific floors of the building. Such voltage surge suppression devices typically include a plurality of metal-oxide varistors (MOVs) connected in parallel between a service power line and a ground or neutral line, or between a neutral line and a ground line.
MOVs are non-linear, electronic devices made of ceramic-like materials comprising zinc-oxide grains and a complex amorphous inner granular material. Over a wide range of current the voltage remains within a narrow band commonly called the varistor voltage. A log-log plot of the instantaneous voltage in volts versus the instantaneous current in amps yields a nearly horizontal line. It is this unique current-voltage characteristic that makes MOVs ideal devices for protection of sensitive electronic circuits against electrical surges, over-voltages, faults or shorts.
When exposed to voltages exceeding their voltage value, MOVs become highly conductive devices that absorb and dissipate the energy related to the over-voltage and simultaneously limit dump current to a neutral line or ground plane. If an over-voltage condition is not discontinued, the MOVs will continue to overheat and can ultimately fail catastrophically, i.e., rupture or explode. Such catastrophic failure may destroy the sensitive electronic equipment and components in the vicinity of the MOVs. The destruction of electrical equipment or components in the electrical distribution system can disrupt power to buildings or floors for prolonged periods of time until such components are replaced or repaired. Moreover, the failure of the MOVs in a surge suppression system may allow the fault condition to reach the sensitive electronic equipment the system was designed to protect.
In U.S. Pat. No. 6,040,971 entitled CIRCUIT PROTECTION DEVICE, issued Mar. 21, 2000, there is disclosed a voltage suppression device for protecting an array of metal oxide varistors in a surge suppression system. The device was operable to drop offline an entire array of MOVs in the event that a voltage surge reached a level wherein one or more of the MOVs in the array might catastrophically fail. In the disclosed device and system, a trigger MOV was designed to have a lower voltage rating than any of the MOVs in the array. Thus, the entire array would drop offline in the event that a surge condition exceeded the voltage rating of the trigger MOV. In some instances, however, it may be desirable to maintain the array of MOVs active and to drop offline only those MOVs sensing a voltage surge exceeding the voltage rating of that particular MOV.
In U.S. Pat. No. 6,430,019 entitled CIRCUIT PROTECTION DEVICE, issued Aug. 6, 2002, and fully incorporated herein by reference, there is disclosed a circuit protection device that includes a non-conductive barrier that moves into a gap between a thermal switch and a voltage sensitive element when the thermal switch moves to an open position. The non-conductive barrier prevents line voltage surges from arcing between the thermal switch and the voltage sensitive element. The non-conductive barrier is biased by extension springs which bear on solder joints connecting the thermal switch to the voltage sensitive element. This condition has the potential to stress the solder joints leading to premature wear out or failure.
The present invention provides a circuit protection device, and a transient voltage surge suppression system incorporating such device, to protect an electrical system from catastrophic failure due to excessive over-voltage conditions or repetitive fault conditions.